1. Field of the Invention
This invention relates generally to railcars having a depressed center section, and more particularly, to a method of making depressed center railcars.
2. Description of the Prior Art
A depressed center railcar is a particular kind of railway freight vehicle which is typically used to transport oversized and often very heavy objects. A depressed bed railcar is a very much like a flat bed railcar with except the center portion of the bed is lower, i.e. depressed, to the tracks than the ends of the car. Hence, the name "depressed center" railcar. Viewed from the side, each end of the depressed center railcar has an S-shaped portion, commonly called a transition member. The high end of each S-shaped transition member has an extended portion which is typically supported on the railroad tracks by a wheeled assembly called a "truck." The "depressed" center section of the railcar is connected between the lower end of the S-shaped transition members and thus is displaced significantly lower to the tracks than the conventional flat bed railcars. Because such oversize objects are also typically very heavy, the depressed center section is normally provided with some amount of built in positive camber. More simply put, the center section of the railcar is provided with a slight upward, i.e., convex, curvature. This camber is provided for pre-stressing the center section so that when a heavy object is loaded onto the center section, the weight causes the curved center section to "flatten" out somewhat, instead of "bowing" as might otherwise occur. If no positive camber were provided, the heavy load could cause the center section to "sink" in the middle which would create a very undesirable stress concentration in the center section which could very likely result in permanent deformation, and possibly failure, of the center section. Thus, the providing a positive camber in the center section is generally necessary to transport heavy loads.
Train tracks frequently wind their way around fixed objects located in proximity to the rails, both man made, such as wayside equipment, and natural, such as natural formations. Additionally, the rail route often passes through tunnels and under bridges. Consequently, in order to transport oversize freight by rail, some means for lowering the oversize object can be necessary for the object to pass through tunnels and under bridges. The depressed center section of these railcars, provided by the S-shaped transition members, permits the rail transport of oversized objects which could not otherwise be transported over the rails by conventional flat bed railcars.
When depressed center railcars were first produced, the entire railcar was cast in as a single structure. As can be imagined this was a very costly method of production. Consequently, such construction methods have generally been abandoned in favor of more modern types of fabrication. Conventionally, the entire car is fabricated from steel plates and structural members, such as beams. The transitions are typically made using rolled steel which is bent to shape and then welded together. The depressed center bed section is typically fabricated from steel beams and plates which are welded together. The railcar is typically constructed as a single unit. In other words, the depressed center section and the transition ends are normally built and connected together at the same time as the rest of the railcar is being manufactured. As the railcar is being built, the generally desired amount of camber in the depressed center section is provided as the transition ends are being connected. For example, a jig can be used in fabricating the entire railcar such that the jig constrains the center section at a desired amount of camber. However, with the method of construction typically used, the amount of camber in the finished car can be inconsistent from car to car. Typically, the some amount of desired camber is provided during fabrication of the railcar. Then, to obtain the precise amount of camber desired, the railcar is typically heated and recambered after fabrication is complete. However, the heating and bending required for recambering can have undesirable effects on the newly fabricated railcar.
The original method of casting the entire railcar is prohibitively expensive. Additionally, the conventional method of fabricating the entire railcar, though more cost effective than casting, also has significant disadvantages. As described above, conventional fabrication methods typically fail to provide the precise amount of desired camber in the finished railcar, and thus require recambering after initial fabrication is completed.
Clearly, cambering the railcar is the source of much difficulty and expense in terms of getting it right and doing it efficiently and consistently. Obviously, it would be much easier to simply build a depressed center railcar having a flat center section. However, as explained previously, some amount of positive camber is typically necessary.
Therefore, there is a need for a method of making depressed center railcars which overcomes the disadvantages of known methods of construction and which provides for consistently and accurately building-in the desired amount of camber without the need for post construction recambering processes. Additionally, such a method should also increase the efficiency of producing precambered depressed center railcars by providing for the use of standardized components.